Method and apparatus for determining the image clarity of a surface

ABSTRACT

Method for determining the image clarity of a surface of a sheet strip or similar, including projection of an image on the surface, detection of a virtual image of the image projected onto the surface and the processing of the detected virtual image into a signal which is presented as a measure for the image clarity of the surface, wherein the image reflected onto the surface is composed of a two-dimensional array of distinct light spots, whereby the processing of the detected virtual image includes the steps of (i) digitizing the detected virtual image with reference to at least one greyness discriminator value, and (ii) determining, for the greyness discriminator value, the number of distinct light spots in the digitized detected virtual image, and from steps (i) and (ii) obtaining a signal measuring image clarity of the whole area of the surface illuminated by the image projected onto the surface.

BACKGROUND OF THE INVENTION

The invention relates to a method for determining the image clarity of asurface of a sheet, strip or similar, comprising reflection of an imageon the surface, detection of a virtual image of the image projected ontothe surface and the processing of the detected virtual image of theimage projected onto the surface into a signal that is presented as ameasure for the image clarity of the surface.

At the same time the invention relates to an apparatus for carrying outsuch a method.

In the refrigerator, washing machine and automotive industries it isdesirable to know the degree of image clarity of a sheet given that thesheet used for the external parts of refrigerators and washing machines,or of cars must be particularly flat and smooth because of the opticalquality after painting. Indeed it has been found that there is a qualityaspect of the sheet used to be found in this image clarity of thesurface of the steel sheet used. In this connection image clarity is theterm known in the above-mentioned industries for describing the degreeof distortion of a virtual image of an image projected onto the surface.In the case of a surface with a relatively poor image clarity, theoutward appearance of the steel sheet used looks somewhat irregularafter painting. This phenomenon is known as orange peel. For reasons ofcost it is desirable at the earliest possible processing stage andpreferably before painting to select the steel sheet for suitability forany exterior parts of white goods or cars which are visible.Consequently it is desirable to have a related measuring method which isreliable and reproducible and which excludes any subjective elements.

Use is known, for example of the reflectometer by ATI Systems Inc.,model 1792, whereby a controlled light beam is projected onto thesurface being examined and the light reflected is received by a receiverunit and converted by a processor unit into a signal that is a measurefor the brilliance of the surface.

It has been found that this known method and apparatus is not verydiscerning and has little value in predicting the orange peel aspect ina sheet to be used for exterior parts. In addition, the value of theinformation obtained with the known method and apparatus depends greatlyon an operator's interpretation of the data obtained.

The object of the invention is to create a reliable method fordetermining the image clarity of a surface which method has a goodprediction value and does not depend on subjective aspects. To this endthe method in accordance with the invention is characterised in that theimage reflected onto the surface is composed of a number of distinctivelight spots and in that the number of distinctive light spots isdetermined from the virtual image detected, which number determines, atleast in part, the magnitude of the signal that is produced as a measurefor the image clarity of the surface.

SUMMARY OF THE INVENTION

In a specific aspect of the method in accordance with the invention itis characterised in that the detected virtual image of the imageprojected onto the surface undergoes an image treatment, whereby thevirtual image is split into a discrete number of image points and perimage point a discrete grey value is determined dependent on the lightintensity of the virtual image detected at the position of that imagepoint, and in that the number of distinctive light spots of the virtualimage is determined by further treatment of the grey values pertainingto the image points.

The simplicity of the treatment of the reflected image is made use of inthat subsequently the virtual image of the image projected onto thesurface is transformed into a binary image by determining a binary valuefor each image point by comparing the grey value determined at eachimage point with an adjustable discriminator value and where the greyvalue of that image point is equal or greater in relation to thediscriminator value, by adjusting the binary value pertaining to thatimage point to a first of the binary values, and where the grey value ofthat image point is smaller in relation to the discriminator value, byadjusting the binary value pertaining to that image point to a second ofthe binary values.

In accordance with a specific aspect of the invention the discriminatorvalue is varied from a lowest to a highest value, and for eachdiscriminator value the pertinent number of distinctive spots isdetermined in the binary image of the virtual image of the imageprojected onto the surface, and the signal that is presented is ameasure of the image clarity of the surface, dependent on therelationship found between the discriminator values and the number ofspots in the binary image.

A very simple embodiment of the method in accordance with the inventionis characterised in that the discriminator value is adjusted and varieduntil the number of distinctive spots in the binary image of the virtualimage of the image projected onto the surface is equal to the number ofdistinctive light spots which compose the image projected onto thesurface, and in that the discriminator value pertaining to this is thesignal that is presented as a measure for the image clarity of thesurface.

The invention also relates to an apparatus for determining the imageclarity of a surface of a sheet, strip or similar, comprising means forreflecting an image on the sheet, a receiver unit for detecting avirtual image of the image projected onto the surface and a processorunit provided with an output unit, which receiver unit is control linkedwith the processor unit, and which apparatus is characterised in thatthe processor unit is a computer which is provided with a program forcarrying out the method in accordance with the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be illustrated in the following by reference to thedrawing of a non-limitative example embodiment.

FIG. 1 shows an arrangement with the apparatus in accordance with theinvention.

FIGS. 2a-f show examples of the result of an image treatment.

FIG. 3 shows a further example of results of use of the invention ondifferent objects.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 the image clarity of the surface (2) of the sheet (1) isdetermined. To this end an image forming element (4) is placed betweenthe sheet (1) and a light source (3), which element (4) consists of agrid of, for example, 36 holes, each spaced approximately 1 mm apart.The image of this grid (4) is reflected by means of the light source (3)onto the surface (2) of the sheet (1), whereby a virtual image (8) of itis detected with a camera (5) and fed into a computer (6). In thiscomputer (6) the necessary, processing takes place on the virtual image(8) of the projected image detected with the camera (5). The computer(6) shows the result of these computations on the output unit (7) asmeasurement signal indicating the image clarity of the sheet surface (2)of the sheet (1).

In FIGS. 2a-f show examples the measurement results are shown of asurface with a relatively good image clarity 2a (see left hand column)and of a surface with a relatively weak image clarity 2b (see right handcolumn). The images 2a and 2b relate to the virtual image (8) of theimage of grid (4) projected onto surface (2) detected with the camera(5). The virtual image (8) detected by camera (5) is fed into thecomputer (6) where processing takes place which means that the detectedvirtual image (8) undergoes an image treatment, whereby the detectedvirtual image is split into a discrete number of image points from whichper image point a discrete grey value is determined dependent on thelight intensity of the virtual image (8) at the position of that imagepoint, and that the number of distinct light spots of the virtual image(8) is determined by further treatment of the grey values pertaining tothe image points.

Subsequently the virtual image (8) of the image projected onto thesurface (2) is transformed into a binary image by determining a binaryvalue for each image point by comparing the grey value determined ateach image point with an adjustable discriminator value and where thegrey value of that image point is equal or greater in relation to thediscriminator value, by adjusting the binary value pertaining to thatimage point to a first of the binary values, and where the grey value ofthat image point is smaller in relation to the discriminator value, byadjusting the binary value pertaining to that image point to a second ofthe binary values.

The result of this treatment is the two images 2c and 2d.

Next, the two images 2c and 2f show the arrangement of one and the samediscriminator value in the case of the surface with the relatively goodimage clarity 2e and of the surface with the relatively poor imageclarity 2f. It can be clearly observed that with one and the samediscriminator value for the relatively good surface 2e as many distinctspots are detected as is projected with grid (4) and the light source(3) onto the surface (2) of the sheet (1) and detected with the camera(5).

With 2f, the poor surface, on the other hand only one spot is detected,which is indeed provided with a number of holes, but this does not leadto a number of distinct spots greater than one. Consequently for thislatter surface the discriminator value must still be re-adjusted andvaried until the number of distinct spots in the binary image of thevirtual image (8) of the image projected onto the surface (2) is equalto the number of distinct light spots which compose the image projectedonto the surface (2). In the end this leads to a binary image that iscomparable with FIG. 2c. The discriminator value pertaining to this is ameasure for the signal that is presented as measure for the imageclarity of the surface.

FIG. 3 shows results obtained by applying the method and apparatus ofthe invention on eight different sheets (a) through (h).

The arrangement of the discriminator value is plotted on the x-axis, andthe y-axis shows the number of distinct light spots that are detected bythe apparatus of the invention based on virtual image (8). The betterquality sheets (d), (e), (f), (g) and (h) show that even at low valuesof the discriminator the number of light spots detected is greater than1, and that with just a slight further increase in the discriminatorvalue, the maximum number of light spots to be distinguished is reached(in this example 36 in due to the number of holes in image grid (4)).The sheets (a), (b) and (c) which possess a poorer image clarity show aclearly distinctive link between the trend of the discriminator valueand the pertinent number of distinct light spots relating to the virtualimage (8). It should also be noted here that, especially the distinctionof the quality of sheet (c) relative to the quality of the sheets (d)through (h) which have a "better" surface takes place very simply andaccurately, with the apparatus and method in accordance with theinvention, while "visually" this is still found to be a problem.

We claim:
 1. A method for determining the image clarity of a surface ofa sheet or strip, comprising projecting an image on to a surface,detecting a virtual image of the image projected onto said surface andprocessing the detected virtual image into a signal which is presentedas a measure of the image clarity of the surface, said image reflectedonto the surface being composed of a two-dimensional array of distinctlight spots, said processing of the detected virtual imagecomprisingdigitizing said detected virtual image with reference to atleast one greyness discriminator value, and determining, for saidgreyness discriminator value, the number of distinct light spots in thedigitized detected virtual image, and from said digitizing and saiddetermining, obtaining a signal measuring image clarity of the wholearea of the surface illuminated by said image projected onto thesurface, said detected virtual image digitizing being performed for eachof a plurality of said discriminator values and determining for eachdiscriminator value the number of distinct spots in the virtual image,and wherein said signal that is presented as a measure of the imageclarity of the surface is dependent on a relationship found between thediscriminator values and the respective numbers of spots in the binaryimage.
 2. The method in accordance with claim 1, comprising finding agreyness discriminator value at which the number of distinct spotsdetermined in said virtual image is equal to the number of distinctlight spots of said image projected onto the surface, said signalpresented as a measure of the image clarity of the surface, beingderived from said greyness discriminator value.
 3. A method fordetermining the image clarity of a surface of a sheet or strip,comprising projecting an image on to a surface, detecting a virtualimage of the image projected onto said surface and processing thedetected virtual image into a signal which is presented as a measure ofthe image clarity of the surface, said image reflected onto the surfacebeing composed of a two-dimensional array of distinct light spots, saidprocessing of the detected virtual image comprisingdigitizing saiddetected virtual image with reference to at least one greynessdiscriminator value by separating said detected virtual image into adiscrete number of image points and for each image point a discrete greyvalue is determined on a light intensity of the virtual image detectedat the position of that image point, and determining, for said greynessdiscriminator value, the number of distinct light spots in the digitizeddetected virtual image by transforming the virtual image into a binaryimage by determining a binary value for each said image point bycomparing the grey value determined at each image point with saiddiscriminator value and assigning a binary value to each said imagepoint in dependence on the result of this comparison, and from saiddigitizing and said determining, obtaining a signal measuring imageclarity of the whole area of the surface illuminated by said imageprojected onto the surface, said digitizing of said detected virtualimage being performed for each of a plurality of said discriminatorvalues and determining for each discriminator value the number ofdistinct spots in the virtual image, and wherein said signal that ispresented as a measure of the image clarity of the surface is dependenton a relationship found between the discriminator values and therespective numbers of spots in the binary image.
 4. The method inaccordance with claim 3, comprising finding a greyness discriminatorvalue at which the number of distinct spots determined in said virtualimage is equal to the number of distinct light spots of said imageprojected onto the surface, said signal presented as a measure of theimage clarity of the surface, being derived from said greynessdiscriminator value.
 5. Apparatus for determining the image clarity of asurface of a sheet or strip comprising means for reflecting an imageonto the sheet, a receiver unit for detecting a virtual image of theimage projected onto the surface and a processor unit provided with anoutput unit, which receiver unit is connected to the processor unit,said processor unit being a computer provided with a programfordigitizing said detected virtual image with reference to at least onegreyness discriminator value, and determining, for said greynessdiscriminator value, the number of distinct light spots in the digitizeddetected virtual image; and from said digitizing and said determining,obtaining a signal measuring image clarity of the whole area of thesurface illuminated by said image projected onto the surface, saiddigitizing of said detected virtual image being performed for each of aplurality of said discriminator values and determining for eachdiscriminator value the number of distinct spots in the virtual image,and wherein said signal that is presented as a measure of the imageclarity of the surface is dependent on a relationship found between thediscriminator values and the respective numbers of spots in the binaryimage.
 6. The apparatus in accordance with claim 5 wherein said computeris additionally programmed to find a greyness discriminator value atwhich the number of distinct spots determined in said virtual image isequal to the number of distinct light spots of said image projected ontothe surface, said signal presented as a measure of the image clarity ofthe surface, being derived from said greyness discriminator value. 7.Apparatus for determining the image clarity of a surface of a sheet orstrip comprising means for reflecting an image onto the sheet, areceiver unit for detecting a virtual image of the image projected ontothe surface and a processor unit provided with an output unit, whichreceiver unit is connected to the processor unit, said processor unitbeing a computer provided with a program fordigitizing said detectedvirtual image with reference to at least one greyness discriminatorvalue, and determining, for said greyness discriminator value, thenumber of distinct light spots in the digitized detected virtual image;and from said digitizing and said determining, obtaining a signalmeasuring image clarity of the whole area of the surface illuminated bysaid image projected onto the surface, said digitizing of said detectedvirtual image comprising separating said detected virtual image into adiscrete number of image points and determining for each image point adiscrete grey value dependent on a light intensity of the virtual imagedetected at the position of that image point, and wherein determiningthe number of distinct light spots in the digitized detected virtualimage comprises transforming the virtual image into a binary image bydetermining a binary value for each said image point by comparing thegrey value determined at each image point with said discriminator valueand assigning a binary value to each said image point in dependence onthe result of this comparison, said digitizing of said detected virtualimage being performed for each of a plurality of said discriminatorvalues and determining for each discriminator value the number ofdistinct spots in the virtual image, and wherein said signal that ispresented as a measure of the image clarity of the surface is dependenton a relationship found between the discriminator values and therespective numbers of spots in the binary image.
 8. The apparatus inaccordance with claim 7, wherein said computer is additionallyprogrammed to find a greyness discriminator value at which the number ofdistinct spots determined in said virtual image is equal to the numberof distinct light spots of said image projected onto the surface, saidsignal presented as a measure of the image clarity of the surface, beingderived from said greyness discriminator value.